Impact of the new drug decitabine on human tumor DNA methylation will be assessed. Down-regulation of tumor suppressor genes & pro-apoptotic genes needed for chemotherapy efficacy may occur due to DNA hypermethylation. Decitabine may decrease DNA methylation & up-regulate gene expression at doses well below maximum tolerated dose (MTD), & effect may plateau at low doses. Six patients treated at each of 5 decitabine dose levels (2.5-20 mg/m2/day days 1-5 & 8-12 of a cycle) in a phase I trial will have tumor biopsies pre & post decitabine. Aim 1: To assess decitabine-induced changes in global DNA methylation, using ALU & LINE assays to compare pre- vs. post-decitabine tumor specimens, and to determine: a) if the effect on DNA methylation is linear at low doses but plateaus as decitabine dose increases; b) the dose at which any such plateau occurs. In later studies (beyond the scope of this grant) in more homogeneous patient populations we will assess if this plateau-level dose is as effective as the MTD: a) in single agent activity vs. specific cancers; b) at reversing resistance to other agents; and c) at demethylating and upregulating expression of silenced tumor suppressor & pro-apoptotic genes. Aim 2: To correlate changes in tumor with changes in peripheral blood mononuclear cell (PBMC) DNA methylation with decitabine. If decitabine impact on PBMC DNA methylation mirrors its impact on tumor DNA methylation, then PBMCs may serve as a useful surrogate for tumor DNA methylation. Aim 3: To use pyrosequencing to conduct preliminary proof-of-principle assessments of impact of decitabine on methylation of selected tumor-suppressor & pro-apoptotic genes. For individual tumors, we will preferentially assess genes frequently reported to be silenced by hypermethylation in that specific tumor type. Impact on individual genes will be confirmed in later studies (beyond the scope of this grant). This preliminary look at individual genes may provide a molecular rationale for future decitabine therapeutic studies in specific relevant tumor types.